I could understand that some pure-class Warlocks might be fanatically devoted to their patron, and thus anything other than continuing to learn how to be the perfect student would violate their principles. But many other Warlock concepts aren't devoted to their patron in the slightest: their pact may be the result of opportunism, a past indiscretion, or even have been inherited (or otherwise acquired unwillingly). How would it compromise any of these characters' principles to work on honing their sorcerous power?
From my standpoint, al a carte multiclassing simply enables one to mechanically represent a wider range of character concepts than are possible with single-classing (or 2e-style multiclassing). Here are some examples that don't work as well without al a carte multiclassing:
- The Midlife Crisis (e.g. the repentant faustian Warlock, the emergent Sorcerer, the crisis-of-faith Cleric)
- The Specialist (e.g. dipping fighter to make a particularly martial War Domain Cleric, dipping rogue to make a wrestling-focused Barbarian, dipping Wizard to make a spell-focused Eldritch Knight) Note: taking a dip to emphasize abilities a character already has doesn't require an IC change in focus--the character is just temporarily training particularly hard on a subset of their existing skills. They don't stop being a "pure" (e.g.) Cleric/Barbarian/EK concept-wise.
- The Character-Defining Mechanic (e.g. skirmishers of any class need the Rogue's Cunning Action, alchemist Wizards needs the Thief's Fast Hands, some shaman Druids may need the Totem Barbarian's Totem Spirit, anyone with a key heirloom magic item may need the Warlock's Blade Pact or Tome Pact)
For reference, almost all of the characters I play are multiclass, in every edition (hybrid in 4th). I usually DM, however, and at my tables more than half of the PCs multiclass. I actively promote multiclassing, however, so this is not an unbiased sample. (I find promoting multiclassing leads to more multi-dimensional characters that are less defined by their race/class combo.)